1. Field Of The Invention
The invention relates generally to interface circuits for industrial automation systems and, more specifically, to a circuit for limiting inrush current through the system during initial turn-on of a clockderived power supply.
2. Description Of The Relevant Art
Reduced to bare essentials, an industrial process may be regarded as having a number of sensors and loads that correspond to input and output variables for a process control computer system. The sensors provide input values representative of the state of the process at a given time; the loads respond to output values, and thereby control various aspects of the process. Typical sensors include relay contacts, proximity switches, and pressure switches. Typical loads include contractor coils of starters for large motors, solenoid valves, relays, lamps, and small motors. A process may have several hundred to several thousand input sensors and loads that must be services at very frequent intervals.
A typical computer system for automating an industrial process contains a number of general and special purpose computers. The system monitors input variables from the process, performs suitable logical manipulations on the inputs, and updates output variables for the process. The computer system is usually organized hierarchically. A host processor, typically a minicomputer or a mainframe, communicates with a number of programmable controllers, each of which communicates with a number of power control subsystems through a number of local processors. A programmable controller is a processor especially designed to handle and perform logical manipulations on a large number of binary inputs on a cyclical basis. The local processors have as their primary function the efficient transfer of data between the power control subsystems and the working memories of the programmable controllers. The power control subsystems provide the interface between the local processors and the various sensors and loads.
A factory automation system, such as described above, must be provided with suitable electrical isolation between the local processor side of the system and the high voltages on the load side of the power control subsystems. The possibility of a short circuit between a motor running from any line voltage (DC or AC, single phase or more) and the programmable controller or local processor directing it, or worse yet the host processor, is horrible to contemplate--in terms of economic cost and danger to human life.
One example of electrical isolation between the local processor side of the system and the high voltages on the load side of the power control subsystems is disclosed in co-pending Application Ser. No. 208,157, filed on June 17, 1988. In that system, a transformer is used for isolation, with clock and data signals being transferred through the transformer from one side of the system to the other. The power for operating the electrical components on the load side of the power control subsystem is derived from the clock signals communicated across the transformer. As with other industrial automation systems, initial turn-on of the system causes a large inrush of current to the electrical components of the system. This shortens the life of the electrical components in the system and creates a risk of damage to them.
Conventional techniques for limiting inrush current include the use of transistors which are selectively turned off and on for variable lengths of time until the desired operating point of the system is reached. However, such systems require complex switching systems while still producing large currents for short periods of time. Thus, such systems do not actually limit the initial rush of current into the system. Furthermore, the current spikes caused by transistor switching generate noise in the system, and this affects reliability and accuracy of the system until the steady operating state is reached.